Lithium electrochemical cells containing an anode, a cathode, and a solvent-containing electrolyte are known in the art. Such cells typically include an anode of metallic lithium or a lithium-inserting compound; a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents; and a cathode of an electrochemical active material, typically a chalcogenide of a transition metal.
During discharge, lithium ions from a lithium metal anode pass through the liquid electrolyte to the electrochemically active material of the cathode, where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode.
When lithium metal anodes are replaced with a carbon anode such as coke or graphite, they are intercalated with lithium ions to form Li.sub.x C. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode, where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode, where it reintercalates into the carbon. Because no metallic lithium is present in the cell, melting of the anode does not occur even under abuse conditions. And because lithium is reincorporated into the anode by intercalation rather than by plating, dendritic and spongy lithium growth does not occur.
Solid, secondary batteries typically comprises several electrochemical cells. The current from each of the cells is accumulated by a conventional current collector so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrochemical cells employed in the battery.
U.S. Pat. No. 5,456,000, which is incorporated by reference in its entirety, discloses the formation of electrochemical cell electrodes and separator elements. The electrodes and separator elements use a combination of a poly(vinylidene fluoride) copolymer matrix and a compatible organic solvent plasticizer to provide battery component layers, each in the form of a flexible, self-supporting film.
An electrochemical cell precursor, such as a rechargeable battery cell precursor, is constructed by means of the lamination of electrode and separator cell elements which are individually prepared. Each of the electrodes and the separator is formed individually, for example by coating, extrusion, or otherwise, from compositions including the copolymer materials and a plasticizer. The materials are then laminated to form an electrochemical cell, as shown in FIG. 1.
In the construction of a lithium-ion battery, for example, a copper grid may comprise the anodic current collector 110. An anode (negative electrode) membrane 112 is formed by providing an anodic material dispersed in a copolymer matrix. For example, the anodic material and the copolymer matrix can be provided in a carrier liquid, which is then volatilized to provide the dried anode membrane 112. The anode membrane 112 is positioned adjacent the anodic current collector 110.
A separator membrane 114 is formed as a sheet of a copolymeric matrix solution and a plasticizer solvent. The separator membrane 114 is placed adjacent the anode membrane 112.
A cathode (positive electrode) membrane 116 is similarly formed by providing a cathodic material dispersed in a copolymer matrix. For example, the cathodic material and the copolymer matrix can be provided in a carrier liquid, which is then volatilized to provide the dried cathode membrane 116. The cathode membrane 116 is then overlaid upon the separator membrane layer 114, and a cathodic current collector 118 is laid upon the cathode membrane.
The assembly is then heated under pressure to provide heat-fused bonding between the plasticized copolymer matrix components and the collector grids. A unitary flexible battery precursor structure is thus produced.
During processing of the battery precursor, a large quantity of a homogeneously distributed organic plasticizer is present in the solid polymeric matrix. Prior to activation of the battery, however, the organic solvent is removed using an extraction process. Extraction is generally accomplished using repeated contact with an extracting solvent such as diethyl ether or hexane, which selectively extracts the plasticizer without significantly affecting the polymer matrix. This produces a "dry" battery precursor substantially free of plasticizer and which does not include any electrolyte solvent or salt. An electrolyte solution containing an electrolyte solvent and an electrolyte salt is imbibed into the "dry" battery polymer membrane structure to yield a functional battery system. The addition of the electrolyte salt and solvent is the "activation" of the battery.
Diethyl carbonate (DEC) is a material which is known for use as an electrolyte solvent. It has a boiling point of 126.degree. C., and a melting point of about -43.degree. C. While DEC is an otherwise excellent electrolyte solvent, the limited useful temperature range it exhibits limits its use. It would be desirable to find a material which is an efficient electrolyte solvent, is electrochemically stable, and which functions at elevated and / or reduced temperatures when compared to DEC. It is also necessary to find electrolyte solvents which are compatible with anode active and cathode active materials, and with the electrolyte salt used.
Propylene carbonate (PC) is another material which has been used as an electrolyte solvent. However, the use of propylene carbonate has been limited with the advent of improved electrode materials. For example, graphite has been used as an electrode material in a variety of lithium ion battery systems due to its high capacity, and to its low and flat voltage curve with respect to lithium metal. Initial efforts to use propylene carbonate in a graphite system met with only limited success. A major problem in using graphite as an electrode in a lithium ion cell with propylene carbonate based electrolyte systems is massive electrolyte decomposition during the first lithiation process. It is generally understood that propylene carbonate decomposes on the surface of graphite (Dey and Sullivan, "The Electrochemical Decomposition of Propylene Carbonate on Graphite", J. Electrochem. Sci., p 222-224, 1970). This results in a very high first cycle capacity loss and reduces the apparent cell capacity of cells using a propylene carbonate-based electrolyte solution. Cycle life is reduced.
Efforts to use graphite and propylene carbonate-based electrolytes within the same battery system have focused on supplying both excess lithium and excess electrolyte within the cell. Such cells can, for example, "front-load" the battery by prelithiating the anode, cathode, and/or electrolyte, so that propylene carbonate decomposition does not deplete electrolyte and cause the battery to fail.
It would therefore be desirable, in view of the limitations of the prior art, to provide an electrolyte solution which exhibits an extended functional temperature range, and/or which reduces or eliminates the decomposition of propylene carbonate during the first lithiation process.